When performing a search across multiple mailboxes, users are added, and will only display the first 20 names selected. This article provides information regarding WebUI search limitations.
We are limited to 20 users in the WebUI search module to avoid excessive processing. This limit is by design. Searches can still be performed by the Import/Export or Mailbox restore tools that include more than 20 users, but the WebUI search is limited in this regard.